1. Field of the Invention
This invention relates generally to dispersing liquids from a printhead, and more particularly, to controlling the temperature of a dispensed liquid at the point of discharge.
2. Description of the Related Art
Dispensing and printing of small quantities of liquids have been done by any of several methods including miniature solenoid valves (microvalves), piezoelectric printheads, continuous-jet printheads, and bubble-jet printheads. In all of these, variable temperature of the dispensed liquid negatively impacted the quality and consistency of the droplet. Precise temperature control of the printed liquid has not been adequately controlled, in part because traditional liquids and applications did not require it. Currently, however, the need to print liquids with greater temperature sensitivity, as well as more sophisticated printing applications, requires more precise control. For example, fluids used in three-dimensional printing can include fluids that are more viscous than or are at the viscosity range of typical ink-jet inks. Also, the viscosity versus temperature slope can be steeper for fluids used in three-dimensional printing. The result of this steeper slope was that even small shifts in temperature resulted in significant shifts in viscosity making it much more difficult to maintain a consistent flow pattern during dispensing.
Further, liquids heated to above ambient temperature were severely impacted by temperature variation because these substances may not be liquid at ambient temperature. Examples of heated liquids include waxy inks, other waxy substances, and solder. For use with hot-melt inks, tank heaters have been used to maintain a melted reservoir of the solder or wax, however, fluctuation in temperature at the point of dispensing continued to negatively impact print quality.
Most commonly in the past, inks, binder liquids and similar liquids have been dispensed at approximately ambient temperature, in the range of 20 to 25° C. However, solenoid valves or microvalves dissipate heat from an internal coil which is powered by an electrical waveform, thus impacting the quality of the printing. Previously, attempts have been made to control the average temperature or external surface temperature of the microvalve body through appropriate thermal management of this heat. Passive thermal management included potting the interior valve region containing the coil with a thermally conductive material to improve heat transfer. Additionally, heat sinks such as fins were used external to the valve. Active temperature control of the block or heat sink where the microvalve was mounted previously included a temperature sensor and a controller. However, in either previously used passive or active temperature control means, there was still some unavoidable and unacceptable temperature rise of the valve coil and variable heat dissipation, causing increased liquid temperature due to these temperature increases. For precise dispensing of fluids, these variations are unacceptable due to the negative impact on the flow pattern of the printed liquid.
FIG. 1 illustrates the heat transfer with conventional microvalve usage. Heat generated inside the valve flowed either outward to the surface of the valve body, labeled Heat Path A, or inward to the liquid passing through the valve, labeled Heat Path B. Typically some combination of heat transfer paths occurred. For example, in the operation of microvalves without temperature control, the external surface temperature of the valve can increase to as much as 15° C. above ambient temperature during steady-state operation. Measured increase in the temperature of the dispensed liquid verifies heat transfer along Heat Path A. Furthermore, the amount of heat transfer from the solenoid into the dispensed liquid was variable depending on the operating history of the microvalve, ambient conditions, duration of operation, and other factors.
Heat from the valve increased the temperature of the dispensed liquid, which then altered the fluid viscosity. Changes in fluid viscosity result in unpredictable shifts to the stream flow regime among the various patterns shown hereinafter. Thus, the techniques described so far for managing the overall temperature of the microvalve body, for example, heat sinks for the microvalve body or control of the surface temperature of the microvalve body, have not completely eliminated inconsistencies and variations of quality of dispensing and printing that are related to temperature variation.
FIGS. 2A-2E illustrate various flow phenomena that can occur during dispensing of droplets. It has been found that changes from an acceptable stream to an unacceptable stream pattern can occur somewhat unpredictably. As noted above, one reason for unpredictable shifts to the stream flow has been due to random variation of the liquid temperature at the point of dispensing, hence the need to more precisely control the temperature of the dispersed liquid is needed.
The valve dispenses in a sequence of many consecutive actuations, typically at 800 Hz, using microvalves and fluid as described herein. Droplet phenomena observed in such dispensing by prior art techniques have been classified into at least the following distinct formations.
Satellites
As illustrated in FIG. 2A, satellite droplets occur when one valve actuation results in more than one produced drop. In such a case, frequently there are two drops produced for each commanded valve actuation, with one of the drops being significantly smaller than the other. In FIG. 2A, there are two similarly sized drops per valve actuation.
Deviations
As illustrated in FIG. 2B, a stream deviation is when a stream of uniform droplets issues from the nozzle but issues in a direction different from the principal axis of the nozzle. Stream deviation often results from a drop of liquid existing on the surface of the nozzle somewhere near the orifice. A stream deviation often appears randomly, but once established, the stream's direction tends to remain consistently off-center. FIG. 2B shows a droplet stream which is deviating to the left.
Stream Splitting
FIG. 2C shows a combination of satellites and stream deviation.
The satellites are much smaller than the main stream and are veering off to the right. The main stream itself has a slight deviation to the left. Stream splitting situations often involve satellite streams with very small diameter droplets. These satellite streams are highly susceptible to air currents and often change their direction.
Drip mode
As illustrated in FIG. 2D, drip mode is a situation in which the nozzle no longer produces a droplet stream. Instead, fluid from many valve actuations accumulates at the nozzle surface in the form of a large drop, and occasionally that very large drop detaches and falls to the print surface. It has been observed that flow, which was in one of the other modes, can transition into drip mode suddenly and without warning.
Acceptable Stream
FIG. 2E illustrates one embodiment of an acceptable droplet stream. The droplet spacing is uniform and constant and no satellites or deviations are present.
The streams in FIGS. 2A, 2B, and 2C would cause dispensing difficulties in various precision applications, for example, three-dimensional printing, because of uneven distribution of binder liquid on the powder bed. Printing with such a stream would yield three-dimensionally printed parts with poor surface finish, poor dimensional control, and poor control of internal microstructure. Drip mode, which is shown in FIG. 2D, would introduce major errors into printed parts, often resulting in the need to discard an entire part being printed.
In addition to temperature variations causing shifts from one flow regime to another, having major and dramatic unacceptable effects on the printed part, it has been found that even if the flow remains within the desirable flow regime, there can be significant changes in the drop velocity due to temperature changes. Drop velocity is important in various precise applications of printing, for example, three-dimensional printing, because placement of drops depends in part on the time of flight of drops from the nozzle to the powder bed. Consistent drop velocity is important in obtaining equidistantly spaced droplets. If the time of flight inadvertently varies during a print job, the dimensions and surface quality of the printed parts are negatively impacted.
FIGS. 3A-3D illustrate one example of a microvalve dispensing a continuous stream of drops over time. FIGS. 3-3D illustrate that even when controllable parameters, including flow rate, are kept constant, droplet velocity may be variable for temperature sensitive materials due to variations in the dispensed liquid's temperature. During the dispensing duration illustrated in FIG. 3, pressure inputs and electrical inputs were held constant, the valve body temperature was controlled as is known in the prior art, but the temperature of the dispensing structure or the nozzle was not controlled.
FIGS. 3A-3D illustrate the results of actual photographed dispensing patterns. FIG. 3A illustrated the initial desired dispensing droplet rate of 1.22 m/s and measured flow rate of 0.49 gms/min at time zero. FIG. 3B illustrates the resulting inadvertent increased dispensing rate of 0.77 m/s and measured flowrate of 0.52 gms/min at 45 minutes. FIG. 3C illustrated the resulting inadvertent additional increased dispensing rate of 0.33 m/s and measured flow rate of 0.53 gms/min at 90 minutes. As shown, the drop velocity unacceptably changed quite noticeably over time until at 90 minutes, the dispensing pattern changed to drip mode illustrated in FIG. 3D, and thus effectively ceased dispensing. These large fractional changes in drop velocity resulted in variable and unpredictable placement of the dispensed drops due to flight time variation. As noted, separate measurements indicate that in this experiment, there were not significant variations in flowrate, only in drop velocity. All of the illustrations of dispensing patterns in FIGS. 2A-2E and 3A-3D represent photographs from experiments having temperature control only applied to the body of the microvalve in accordance with prior art practice.
As illustrated in FIGS. 2-2E and 3-3D, varying liquid temperature at the point of dispensing the liquid negatively impacts the consistency, repeatability, and quality of the droplet size; as well as drop velocity, spacing, and positioning.